1. Field of the Invention
The present invention relates generally to a thin-film optical function element, and more particularly to such a thin-film optical function element adapted to separate two different optical modes which have field patterns whose vibrating directions are perpendicular to each other. This optical function element is suitably used for an optical head for a magnetooptical data storage disk, wherein a light beam produced by a light source is guided through a thin-film optical waveguide toward the magnetooptical disk, such that the light beam maintains a constant plane of polarization, and wherein one or both of the two transverse modes of the light beam reflected by the disk is/are converted into an electric signal or signals.
2. Discussion of the Prior Art
There is shown in FIG. 9 a conventional optical head for writing and reading information on and from a magnetooptical memory or storage disk. This optical head is adapted such that a laser beam emitted from a semi-conductor laser element 41 is collimated by a collimator lens 42, transmitted through a half mirror 43, and focused by an objective or converging lens 44 on the surface of a magnetooptical disk 45. The laser beam reflected by the surface of the disk 45 is reflected by the half mirror 43 toward a polarizing beam splitter 46. A component P of the laser beam incident upon the beam splitter 46 is transmitted through the beam splitter 46, while a component S of the beam is reflected by the beam splitter 46. These two components P and S are received by respective two light-sensitive elements 47, 48. Information stored at a specific reading spot on the magnetooptical disk 45 which is irradiated with the laser beam is read by obtaining a difference between output levels of the two light-sensitive elements 47, 48. The principle of this information reading or retrieving operation is illustrated in FIG. 10. If the laser beam was reflected by the surface of the disk 45 without being influenced by the information stored thereon, the plane of polarization of the reflected laser beam incident upon the polarizing beam splitter 46 would be inclined at 45.degree. with respect to the axes of the beam splitter 46 along which components P and S are polarized. The plane of polarization of the reflected laser beam from the disk 45 is rotated by an angle of +.theta.k or -.theta.k with respect to the plane of polarization of the incident laser beam, depending upon the direction in which the irradiated reading spot is magnetized, i.e., depending upon the stored data "0" or "1". When the plane of polarization of the reflected laser beam is rotated by +.theta.k, a difference between the outputs of the two light-sensitive elements 47, 48 which receive the P and S components of the reflected laser beam is equal to (S.sub.+ -P.sub.+). When the plane of polarization is rotated by -.theta.k, the above difference is equal to (S.sub.- -P.sub.-)=-(S.sub.+ -P.sub.+). Thus, the differential outputs .+-.(S.sub.+ -P.sub.+) corresponding .+-..theta.k are obtained by a differential amplifier which receive the outputs of the light-sensitive elements 47, 48. When information is written on the disk 45, the intensity of the produced laser beam is increased to heat the relevant spot on the disk 45 to a temperature in the neighborhood of the Curie point or above the compensation temperature of the material of the disk. The heated spot is then magnetized in the appropriate direction corresponding to the information to be written, within an externally produced magnetic field. When the information on the disk is erased, the relevant spots on the disk are unidirectionally magnetized in the predetermined direction.
The conventionally available optical head as described above uses optical components made of glasses or other suitable materials, such as lenses, half mirror and prism. These optical components require fine adjustments of their optical axes. Further, the optical head using these optical components tends to be large-sized and heavy, and consequently the overall arrangement of the information reading and writing system tends to be large-sized. Moreover, the relatively heavy optical head leads to a comparatively long access time.